Photoplethysmogram (PPG) sensors can be used to determine physiological information of a user. In a basic form, a PPG device can employ one or more light sources and one or more light detectors. When a PPG sensor unit is positioned such that the light source(s) and the light detector(s) are placed against or in proximity to the skin of a user, the light source(s) can emit light to illuminate the user's skin. The light detector(s) can measure light incident on the light detectors to be used to determine the amount of light from the light source(s) that reaches the detector(s) (e.g., light that has transmitted, reflected, and/or scattered and exited the user's tissue). The amount of light measured by the light detectors (e.g., in the form of one or more signals) can vary based on the amount of light absorbed by the tissue of the user. The device can monitor this absorption to determine one or more physiological parameters, such as a heart rate. A relative change in the blood volume in the body's blood vessels can occur as part of the cardiac cycle (e.g., a repeated sequence of events of during which the blood vessels contract and/or relax to pump blood through the body). These relative changes may result in changes in the amount of light absorbed by the tissue of the user. These relative absorption changes may be measured by the PPG sensor and analyzed to provide measure(s) of one or more aspects of the cardiac cycle. As an example, a PPG sensor unit can measure the timing and/or characteristics of individual heartbeats. The light detector(s) can convert the measured light into an electrical signal indicative of the intensity thereof. For example, the electrical signal can be converted into a heart rate signal, which can include the information associated with timing and/or characteristics of the individual heartbeats.
In some instances, one or more aspects of a user's heartbeat (and/or a pattern of heartbeats) may differ from what would typically be expected of the user under a given set of conditions. For example, a heart rate when the user is at rest can fall between 60 and 100 beats per minute, but factors such as stress or anxiety may cause the user's heart rate to exceed their typical resting heart rate. In other instances, certain conditions, including heart arrhythmias such as atrial fibrillation, may cause irregular heart rate patterns, such as an increase in the variance in beat-to-beat timing over time. When using a sensor to monitor a user's cardiac patterns, it may be desirable for the measured signal of heartbeats to reflect irregular pulse rhythms, fast heart rates, etc., when they may occur. Some devices, however, may not be able to detect the irregularity in a signal, and other devices may not be able to distinguish between irregularity due to the user's physiological condition and noise (e.g., motion artifacts).